Origin of Modern Astronomy

Early history of astronomy  Ancient Greeks Used philosophical arguments to explain natural phenomena Most ancient Greeks held a geocentric (Earth-centered) view of the universe Earth was a motionless sphere at the center of the universe

Ancient Greeks (cont’d) Stars were on the celestial sphere Transparent, hollow sphere Celestial sphere turns daily around Earth Seven heavenly bodies (planetai) Changed position in sky The seven wanderers included the Sun Moon Mercury through Saturn (excluding Earth)

Aristarchus ( B.C.) was the first Greek to profess a Sun-centered, or heliocentric, universe Planets exhibit an apparent westward drift –Called retrograde motion »Occurs as Earth, with its faster orbital speed, overtakes another planet

Explain the reason for the observed motion of Mars relative to the background of stars.

Ptolemaic system A.D. 141 Geocentric model To explain retrograde motion, Ptolemy used two motions for the planets Large orbital circles, called deferents, and Small circles, called epicycles

The universe according to Ptolemy, second century A.D. Figure 21.4 A

Retrograde motion as explained by Ptolemy

Birth of modern astronomy 1500s and 1600s 1. Nicolaus Copernicus ( ) Concluded Earth was a planet Constructed a model of the solar system that put the Sun at the center, but he used circular orbits for the planets

Early history of astronomy 2. Tycho Brahe ( ) Tried to find stellar parallax – the apparent shift in a star's position due to the revolution of Earth Did not believe in the Copernican system because he was unable to observe stellar parallax

Early history of astronomy 3. Johannes Kepler ( ) Planets revolve around the Sun Three laws of planetary motion Orbits of the planets are elliptical Planets revolve around the Sun at varying speed There is a proportional relation between a planet's orbital period and its distance to the Sun (measured in astronomical units (AU’s) – one AU averages 93 million miles)

Kepler’s law of equal areas Figure 21.10

Early history of astronomy 4. Galileo Galilei ( ) Supported Copernican theory Constructed an astronomical telescope in 1609 Four large moons of Jupiter Planets appeared as disks Phases of Venus Features on the Moon Sunspots

Early history of astronomy 5. Sir Isaac Newton ( ) Law of universal gravitation Proved that the force of gravity, combined with the tendency of a planet to remain in straight-line motion, results in the elliptical orbits discovered by Kepler

Constellations  Configuration of stars named in honor of mythological characters or great heroes  Today 88 constellations are recognized  Constellations divide the sky into units, like state boundaries in the United States  The brightest stars in a constellation are identified in order of their brightness by the letters of the Greek alphabet – alpha, beta, and so on

Positions in the sky  Stars appear to be fixed on a spherical shell (the celestial sphere) that surrounds Earth  Equatorial system of location A coordinate system that divides the celestial sphere Similar to the latitude-longitude system that is used on Earth's surface

Positions in the sky Two locational components Declination – the angular distance north or south of the celestial equator Right ascension – the angular distance measured eastward along the celestial equator from the position of the vernal equinox

Astronomical coordinate system on the celestial sphere Figure 21.16

Navigation Stars Astronauts use “nav stars” (navigation stars) for space navigation. They are trained to identify various starfield patterns and the navigation stars that are found within the patterns. The measuring of the position of the navigation stars can help to determine one’s current position in space or help to chart a course to a different location. Charting a course to a body in space is no easy matter, especially since, in space, things are always moving! Take the example of the moon. If one were to leave from the Earth on a trip to the moon, and charted a course directly towards the moon on the day of departure, the ship would not reach the moon! By the time the ship reached the correct distance to the moon, the moon would have moved in its orbit around the Earth and would no longer be in the same place! In order to reach the moon, the navigator must be able to predict the future position of the moon for the time that it will take to reach the proper distance, and then chart a course to that predicted position by determining the nav stars in that area.

Given that the moon moves eastward through the zodiac at the approximate rate of 54 minutes of right ascension per day, determine the following information for each of the problem: Your ship departs Earth’s orbit on September 4. The moon is located at 2 hours 25 minutes. Your expected arrival date is September 9. a)The name of the starfield (constellation) for the position of the moon for the first date given. b) The approximate position of the moon as measured in right ascension (using the units of hours and minutes) for the day of arrival. c) The appropriate nav star to use for the trip (the one closest to the predicted position of the moon for the arrival date). Note: it should be in the zodiac. List the nav star’s number.

Earth motions  Two primary motions Rotation Turning, or spinning, of a body on its axis Two measurements for rotation Mean solar day – the time interval from one noon to the next, about 24 hours Sidereal day – the time it takes for Earth to make one complete rotation (360º) with respect to a star other than the Sun – 23 hours, 56 minutes, 4 seconds

The difference between a solar day and a sidereal day Figure 21.19

Earth motions  Two primary motions Revolution The motion of a body, such as a planet or moon, along a path around some point in space Earth's orbit is elliptical Earth is closest to the Sun (perihelion) in January Earth is farthest from the Sun (aphelion) in July The plane of the ecliptic is an imaginary plane that connects Earth's orbit with the celestial sphere

Earth motions  Other Earth motions Precession Very slow Earth movement Direction in which Earth's axis points continually changes Movement with the solar system in the direction of the star Vega Revolution with the Sun around the galaxy Movement with the galaxy within the universe

Precession of Earth Figure 21.22

Motions of the Earth-Moon system  Phases of the Moon The relative positions of the Sun, Earth, and Moon constantly change Lunar phases are a consequence of the motion of the Moon and the sunlight that is reflected from its surface

Phases of the Moon Figure 21.23

Motions of the Earth-Moon system  Lunar motions Earth-Moon Synodic month Cycle of the phases Takes 29 1/2 days Sidereal month True period of the Moon's revolution around Earth Takes 27 1/3 days

Motions of the Earth-Moon system  Lunar motions Earth-Moon The difference of two days between the synodic and sidereal cycles is due to the Earth-Moon system also moving in an orbit around the Sun Moon's period of rotation about its axis and its revolution around Earth are the same, 27 1/3 days Causes the same lunar hemisphere to always face Earth

The difference between the sidereal month and the synodic month Figure 21.24

Motions of the Earth-Moon system  Eclipses Simply shadow effects that were first understood by the early Greeks Two types of eclipses Solar eclipse Moon moves in a line directly between Earth and the Sun Can only occur during the new-Moon phase

Solar eclipse Figure 21.25

Motions of the Earth-Moon system  Eclipses Two types of eclipses Lunar eclipse Moon moves within the shadow of Earth Only occurs during the full-Moon phase For any eclipse to take place, the Moon must be in the plane of the ecliptic at the time of new- or full-Moon

Lunar eclipse Figure 21.26

Motions of the Earth-Moon system  Because the Moon's orbit is inclined about 5 degrees to the plane of the ecliptic, during most of the times of new- and full-Moon the Moon is above or below the plane, and no eclipse can occur The usual number of eclipses is four per year